The invention relates to a device for fibre optic temperature measurement with an optical fibre, which has a radiation coupling in area and a detector-associated radiation coupling out area.
A device of this kind is known from EP 0 898 158 A2, which describes a high temperature pyrometer, which is in particular suitable for measuring temperatures in a gas turbine. In the case of this pyrometer infrared (IR) radiation is coupled into an optical fibre in a high temperature area of the measuring arrangement. The optical fibre leads to a detector, which is located in an area where moderate temperatures prevail.
According to EP 0 898 158 A2, the optical fibre is made from fused quartz or sapphire crystal. Such optical fibres are heat resistant up to temperatures of approximately 1100° C. or 1900° C., respectively, and are therefore suitable for use in the high temperature range. In addition, such optical fibres are transparent in a spectral range of a heat source with temperatures in the high temperature range and therefore suitable for high temperature measurements.
However, such optical fibres have a very high absorption for an IR radiation with a wavelength higher than typically 4 μm. Therefore these optical fibres are unsuitable for transmitting IR radiation in a spectral range of a heat source with temperatures in the low temperature range, i.e. typically −100° C. to +400° C. Thus, pyrometers according to EP 0 898 158 A2 are unsuitable for temperature determination in the low temperature range.
EP 1 180 669 A1 discloses a device used for temperature determination in the low temperature range. This device is a measuring arrangement for pyrometric temperature determination of a cooking vessel. A detector is also provided, which is placed in a cooler area removed from a heat source. For transmitting thermal radiation to the detector tubular IR lines are provided, whose inner faces are IR-reflecting, e.g. gold-coated.
Such tubular IR lines suffer from the disadvantage that they are not very flexible, so that the measuring arrangement is only suitable for a limited number of measuring tasks. In addition, the IR-reflecting inner faces are subject to ageing, which leads to a change to the reflection factor of the walls and therefore the transmission of the IR line. In turn this leads to a falsification of the measured value. It is also disadvantageous that the radiation losses in such IR lines are relatively high, which makes difficult measurements over longer distances.
The object of the invention is to provide a device for fibre optic temperature measurement with high measurement precision, which is suitable for the low temperature range, particularly approximately −100° C. to +400° C. and in which the actual detector can be positioned in a cool area remote from the measurement point.